1. Technical Field
The present invention relates to an image recording device of an electrophotographic type for transferring a toner image on a photosensitive body onto paper by means of a transferring roller to which a voltage is applied.
2. Background Art
Generally, an image recording device of an electrophotographic type is provided with a charger for uniformly charging the surface of a photosensitive drum, an exposer for forming an electrostatic latent Image by irradiating light on the surface of the photosensitive drum, a developer for forming a toner image by adhering toner to the electrostatic latent image and a transfer unit for transferring the toner image onto paper. A conventional transfer unit is provided with a discharge wire for carrying out corona discharge. In transferring the toner image, a predetermined high voltage is applied to the transferring roller which is located on the opposite side of the photosensitive drum over the paper by corona discharge from the transfer unit. Further, the toner image on the photosensitive drum is attracted toward the paper based on a potential difference between the voltage of the photosensitive drum and the voltage of the transfer unit and is transferred onto the paper.
Corona discharge is accompanied by generation of ozone, and in recent years, in view of environmental concerns, an image recording device that does not generate ozone, more specifically, a technology for transferring a toner image without using corona discharge has been requested. To meet the request, a transfer unit is designed to have a roller made from urethane foam or the like and a predetermined high voltage is applied to the transferring roller so that a potential difference is produced between a photosensitive drum and a transferring roller which are arranged to nip the paper.
However, the transferring roller constructed of urethane foam or the like has a resistance value which changes in accordance with changes in the surrounding environmental conditions such as temperature, humidity or the like. Therefore, even if a voltage predetermined under a particular condition as an optimum voltage (optimum voltage for transferring a toner image) is applied to the transferring roller in order to set the voltage of the transferring roller to the predetermined voltage, the environmental conditions of the transferring roller may have already changed when the voltage is actually applied to the transferring roller. Since the optimum voltage varies with the environmental conditions, always applying the same predetermined voltage to the transferring roller would result in insufficient adhesion of the toner image to the paper and deterioration of the quality of the transferred image.
Another conventional image recording device is disclosed in Japanese Patent Application, Laid-Open Publication No. 8-123222. This image recording device adjusts the application voltage depending upon various conditions, but the resistance of the transferring roller and environmental temperature are not among these conditions.
Still another conventional image recording device is disclosed in Japanese Patent Application, Laid-Open Publication No. 4-275583. This image recording device determines a voltage Vt' to be applied to a transfer member by the equation of Vt'=aVt+b where Vt is the voltage of the transfer member when no image transfer operation is conducted, and "a" and "b" are compensation coefficients. The voltage Vt is compared with a reference value and the coefficients "a" and "b" are changed based on a difference between the voltage Vt and the reference value such that the application voltage Vt' is adjusted according to changing conditions.